Keep exercising caution on steep, unsupported slopes at upper elevations. Wind slabs reactivity may persist longer as they overlie a widespread persistent weak layer.
Strong winds will continue to transport the recent storm snow, which overlies a layer of buried surface hoar, and in some areas a thin crust. Due to the weak layer below, slabs reactivity may persist where the snow surface remains dry and crust-free at higher elevations. Wind slabs are most likely to be reactive in lee and cross loaded features.
A persistent slab problem formed by a crust down 140-200 cm has created a low likelihood, high consequence scenario that is difficult to forecast. This deeply buried weak layer has produced a number of large and surprising avalanches over the past two weeks. This problem is most likely to be triggered from thin or variable snowpack depth on wind-affected features, ridge crests, and near rocky outcroppings. The same feature or path can slide repeatedly when the persistent weak layer is reloaded with new snow.
SUNDAY NIGHT: Cloudy, no precipitation, strong westerly wind, treeline temperatures around -4 C, possible temperature inversion with alpine temperatures of -1, freezing level rising to 1000 m.
MONDAY: Flurries up to 5 cm, moderate to strong southwest wind, treeline temperatures around -2 C, freezing level around 1000 m.
TUESDAY: Flurries, light northwest wind, treeline temperatures around -3 C, freezing level rising to 500 m.
WEDNESDAY: A mix of sun and clouds, light variable wind, treeline temperatures around -6 C, freezing level returning to valley bottom.
No new avalanches were reported in the last 24h.
Thursday, large avalanches (size 2.5) with impressive propagation were triggered by strong solar radiation and warm temperatures (+5 C at treeline). Several loose wet avalanches were also observed at lower elevations along with significant pinwheeling. While in motion, some of these loose avalanches pulled large storm slabs, scouring avalanche paths to the ground in places.
Wednesday, a widespread avalanche cycle occurred, with numerous sizes 1-2 storm slabs avalanches and one size 3 avalanche near Ymir Mountain. These avalanches were 20-40 cm thick, and in many cases were suspected to have failed on a recently buried surface hoar layer (see photos in the MIN post).
We have not heard reports of persistent slab avalanches since last weekend when we saw numerous sizes 2 to 3 avalanches releasing on the early December facet/crust layer. Prior to these avalanches, we were seeing a regular pattern of large destructive avalanches on this layer at the start of the month. These avalanches mostly occurred on thin, wind-affected slopes near ridge tops.
Winds have redistributed the dry alpine snow on lee slopes and open areas at treeline. Around 1900 m and below, a crust is now capping the dense 10-30 cm of recent snow which has settled significantly with the past mild temperatures. The crust varies from thin/breakable to thick/supportive, according to the aspects and elevation. Below the recent snow, a layer of feathery surface hoar and melt-freeze crusts are found, which seems to be still reactive in isolated areas. Lower elevations have gone through a melt-freeze cycle and snow can still be moist from warming/rain event.
The early December crust/facet layer has been responsible for sporadic but very large, persistent slab avalanches over the past month. The crust is now buried 120-200 cm deep except in thin, wind-affected areas near ridgetops where most of the recent avalanches have been triggered.
